In a fuel injector, the displacements of a valve member, or needle, between an open position and a closed position enable, or forbid, fuel injection through spray holes provided in the nozzle body of the injector. The needle is an elongated shaft-like member extending from a head portion, protruding in a control chamber, to a pointy extremity provided with a moving seating face that cooperates with a fixed seating face integral to the nozzle body. The needle is slidably guided between an upper guide and a lower guide arranged in the nozzle body and, in closed position the moving seating face is in sealing contact against the fixed seating face closing fluid communication to the spray holes and thus forbidding fuel injection and, in open position the moving seating face is lifted away from the fixed seating face thus opening said fluid communication and enabling fuel injection through the spray holes.
The needle moves under the influence of fuel pressure difference between the pressure in the control chamber that generates on the needle a closing force and, pressure on the pointy extremity of the needle that generates an opposed opening force.
The pressure difference alternates as the pressure in the control chamber raises to a first level where the closing force is predominant or, drops down to a second level where the opening force becomes predominant. The control chamber is fed with fuel at high pressure wherein, the pressure variation depends upon a control valve to open or to close a spill orifice enabling fuel to exit the control chamber and flow back to a return circuit toward a low pressure reservoir.
It is now known that major improvement in the control of fuel injection equipment and of the injection event is obtained with a so called closed-loop control method. In such method, executed by an electronic control unit (ECU) that controls the all operation of the fuel injection equipment and in particular the control valve of the fuel injector, the fuel injector is provided with close loop means enabling for an electrical signal to be measured at a specific value when the needle gets in closed position. In other disclosed embodiments, the signal can also take a specific value when the needle is in fully open position. Such close loop means typically comprise the electrical insulation of the needle relative to the nozzle body, to the exception of the moving seating face and fixed seating face that are electrically conductive so that, the needle and nozzle body cooperate as an electrical switch part of an electrical circuit which is closed when the needle is in closed position and which is open when the needle is either in ballistic mode or in fully open position. In the alternative above mentioned, the electrical circuit is only open in ballistic mode as it closes again when the needle reaches the fully open position. Consequently, a 0-1 step signal can be measured and entered in an electronic control unit (ECU) controlling the fuel injection equipment as a feedback signal which is taken into account in the parameters of the control algorithm of the fuel injection equipment.
Such close loop means have been disclosed following various embodiments in applications PCT/EP2014/073662, FR3013080 and FR1457078.
Nowadays, the demands are constantly rising and the requests for more stringent performances and improved anti-pollution behavior are required. Indeed under very high pressure and very fast needle displacements, the needle member can slightly bend or be angled relative to the nozzle body axis and, when the needle reaches the closed position an initial contact between the two seating faces occurs before they get in sealing abutment closing the fluid communication toward the spray holes. Another reason for this effect can also be a slight misalignment between the upper guide and the lower guide, said misalignment being related to manufacturing tolerances. When said initial contact occurs, the electrical circuit closes and a closing signal is sent to the ECU while indeed, the fluid communication is not yet perfectly sealed, the needle continuing its closing displacement in sliding with friction against the inner wall of the nozzle body. This inaccuracy of needle closed position allows for fuel droplets to form and exit the spray holes when this should not happen.
Similarly, when an injection event is commanded by the ECU, the needle member starts to lift off but, during the initial stage of the move, the two seating faces maintain a temporary partial contact while the fluid communication already opens.
Both in closing and in opening mode, the ECU receives an incorrect signal corresponding to a closed position of the needle while indeed, it is still, or already, open.